The Collaborative International Dictionary of English v.0.48:

Agate \A*gate"\, adv. [Pref. a- on + gate way.]
   On the way; agoing; as, to be agate; to set the bells agate.
   [Obs.] --Cotgrave.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Agate \Ag"ate\, n. [F. agate, It. agata, L. achates, fr. Gr. ?.]
   1. (Min.) A semipellucid, uncrystallized variety of quartz,
      presenting various tints in the same specimen. Its colors
      are delicately arranged in stripes or bands, or blended in
      clouds.
      [1913 Webster]

   Note: The fortification agate, or Scotch pebble, the moss
         agate, the clouded agate, etc., are familiar varieties.
         [1913 Webster]

   2. (Print.) A kind of type, larger than pearl and smaller
      than nonpareil; in England called ruby.
      [1913 Webster]

   3. A diminutive person; so called in allusion to the small
      figures cut in agate for rings and seals. [Obs.] --Shak.
      [1913 Webster]

   4. A tool used by gold-wire drawers, bookbinders, etc.; -- so
      called from the agate fixed in it for burnishing.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Chalcedony \Chal*ced"o*ny\ (k[a^]l*s[e^]d"[-o]*n[y^] or
   k[a^]l"s[-e]*d[-o]*n[y^]; 277), n.; pl. Chalcedonies
   (-n[i^]z). [ L. chalcedonius, fr. Gr. CHalkhdw`n Chalcedon, a
   town in Asia Minor, opposite to Byzantium: cf. calc['e]doine,
   OE. calcidoine, casidoyne. Cf. Cassidony.] (Min.)
   A cryptocrystalline, translucent variety of quartz, having
   usually a whitish color, and a luster nearly like wax.
   [Written also calcedony.]
   [1913 Webster]

   Note: When chalcedony is variegated with with spots or
         figures, or arranged in differently colored layers, it
         is called agate; and if by reason of the thickness,
         color, and arrangement of the layers it is suitable for
         being carved into cameos, it is called onyx.
         Chrysoprase is green chalcedony; carnelian, a flesh
         red, and sard, a brownish red variety.
         [1913 Webster]

WordNet (r) 3.0 (2006):

agate
    n 1: an impure form of quartz consisting of banded chalcedony;
         used as a gemstone and for making mortars and pestles

Easton's 1897 Bible Dictionary:

Agate
   (Heb. shebo), a precious stone in the breast-plate of the high
   priest (Ex. 28:19; 39:12), the second in the third row. This may
   be the agate properly so called, a semi-transparent crystallized
   quartz, probably brought from Sheba, whence its name. In Isa.
   54:12 and Ezek. 27:16, this word is the rendering of the Hebrew
   cadcod, which means "ruddy," and denotes a variety of minutely
   crystalline silica more or less in bands of different tints.

     This word is from the Greek name of a stone found in the river
   Achates in Sicily.